1. Field of the Invention
The invention relates to a two-stroke internal combustion engine and more particularly to an arrangement for varying the compression ratio of such and the area of an exhaust port of a cylinder of such.
2. Description of the Related Art
In a ported two-stroke engine the skirt of the piston serves to close the ports in the cylinder, one or more of these ports serving to provide a passage for the injection of a fresh charge of air or a fuel/air mixture to the cylinder and one or more other ports serving to provide an exhaust output for the combusted gases. The inlet ports and exhaust ports are arranged in the cylinder so that on downward movement of the piston the exhaust ports are uncovered first, the high pressure differential between the gases in the cylinder and atmospheric pressure causing the combusted gases to flow out of the cylinder into an exhaust passage which leads to an exhaust pipe which delivers the gases to the atmosphere. On further downward motion of the piston the inlet ports are uncovered enabling a fresh charge of pressurised fuel/air mixture to be delivered to the cylinder for combustion. The pressurised delivery of gas also serves to force combusted gases from the cylinder, a process known as scavenging.
In traditional two-stroke engines, the time during which both the inlet and the outlet ports are uncovered is controlled solely by the motion of the actual piston itself, the only means of closing the apertures being provided by the piston. When the piston moves towards the top of the cylinder it closes first the inlet ports and secondly the exhaust ports.
In EP-0526538 there is described a two-stroke engine comprising a moveable shutter for varying the effective area of the exhaust port. The shutter varies the effective area cyclically in a timed relationship to the reciprocal motion of the piston within the cylinder. Sensors measure operating characteristics of the engine and a control unit processes signals generated by the sensors and controls the motion of the shutter accordingly. The shutter is operated by a transmission mechanism which oscillates the shutter between a first position in which the exhaust port has a first effective area and a second position in which the exhaust port has a second smaller effective area. The transmission mechanism is connected to a crankshaft connected to the piston of the engine and comprises a plurality of interconnected links. The shutter is in or close to the second position thereof when the piston passes the shutter when moving from the bottom dead centre position thereof to the top dead centre position thereof. The first position of the shutter is varied by the control unit with changes in sensed operating characteristics of the engine. The shutter is in or close to the first position when the piston passes the shutter when moving from the top dead centre position thereof to the bottom dead centre position thereof. The control unit varies the first position of the shutter with change in sensed operating characteristics to advance or retard the opening of the exhaust passage. The control unit varies the first position of the shutter by varying the amplitude of oscillation of shutter travel between the first and second positions thereof. The control unit decreases the shutter movement to retard opening of the exhaust passage. The second position of the shutter is constant for all engine operating conditions. An electro-mechanical device is connected to one of the interconnected links, the electro-mechanical device being controlled by the control unit to alter the configuration of the interconnected links to vary the cyclical motion of the shutter.
The “effective area” of the exhaust port is the area through which gases may pass to the exhaust passage. The exhaust port itself will have a fixed area, being an aperture machined in the side of the engine's cylinder. The shutter acts to vary the effective area of the exhaust port.
The engine of EP0526538 enables the point at which the combined gases can flow from the cylinder in each cycle to be varied with varying engine characteristics by alteration of the first position of the shutter, (i.e. the position in which the exhaust port has the largest effective area).
Recently to achieve cleaner combustion, engines have been run with Homogeneous Charge Compression Ignition (HCCI). This involves introducing gasoline into a mixture of charge air and combusted gases and then allowing the formation of a roughly homogeneous mixture which ignites on compression (without a spark). The combustion process requires retention of heat and combusted gases in a cylinder.
In EP 0526538 concern was expressed about the retention of combusted gases as a result of the use of the shutter; this was felt undesirable.